The Seventh Secret: by Irving Wallace.

A painting that turns up in Berlin gives a clue to a startling mystery, since the painter is supposed to have died in wwII but the subject of the painting features a building that was finished much later, and yet the painting is obviously made from life, not a plan or sketch. And to an art expert, a painting is what a fingerprint is to fingerprint experts or a signature is to experts in that area. Unmistakable proof.

It leads various people investigating into an ally that finishes with a serious question - what do you do if you are perfectly innocent, but nevertheless you and your progeny are not only remnants of a dark past but far more - a possibility of the past revived in future, unthinkable for survival of humanity and of anything and everything that is achieved by human civilisation, anything and everything that is good in humanity?